Method and apparatus for removing amplitude modulation



MODULATION July 13; 1943. V

E. H. LANGE METHOD AND APPARATUS FOR REMOVING AMPLITUDE- Filed Oct. 25, 1941 2 Sheets-Sheet 1 Job r we

INVENTOR.

I i i eas as FIGS July 13, 1943. E. H. LANGE METHOD AND APPARATUS FOR REMO VING AMPLITUDE-MODULATION Filed Oct. 25, 1941 2 Sheets-Sheet 2 FIG.6

INVENTOR.

Patented uly 13,

METHOD AND APPARATUS FOR REMOVING AIMPLITUDE MODULATION Edward H. Lange, Baltimore, Md.

Application October 25, 1941, Serial No. 416,43

11 Claims.

This invention pertains to a method for removing amplitude modulation from an alternating voltage, for example an alternating carrier-frequency voltage having amplitude-modulation, and with particular reference to a frequencymodulated carrier voltage having undesired amplitude-modulation, and t9 several simplecircuit means carrying out the method.

The principal object of this invention is to provide a method and several forms of apparatus applying the method, for removing amplitudemodulation of an alternating voltage. Another object is to provide a simple method and means for removing a high percentage of amplitudemoclulation, that is for removing amplitude-modulation from an alternating carrier-voltage having a high percentage of amplitude-modulation. Still another object is to provide a simple method and apparatus for annulling amplitude-modulation present in a high percentage, without substantial reaction upon and modification of carrier wave-form.

These objects, and others, and important features of this invention are hereinafter further pointed out in the specification and appended claims, and will be better understood by reference to the drawings.

Referring to the drawings,

Fig, la illustrates a conventional stage of thermionic amplifier suitable for linkage with the devices of this invention, for applying amplitude modulated voltage.

Fig. 1 illustrates a circuit with a single thermionic tube for carrying out the method of this invention.

Fig. 2 shows an amplitude-f'modulation removing circuit employing two thermionic tubes, and is a modification of Fig. 1.

Fig. 3 illustrates an alternative form of linkfor interconnecting a stage such as is shown in Fig. 1a with the clrcuitsof Fig. l or Fig. 2.

Fig. 4 illustrates an alternating carrier-voltage having amplitude-modulation.

Fig. 5 illustrates a typical characteristic of grid transconductance in relation to negative grid bias employed in the circuit of Fig. 1 and Fi 2.

Fig. 6 illustrates an amplitude-modulation removing circuit employing the method of th s invention in a further modification of apparatus.

Fig. 7 illustrates another relationship between cathode-plate conductance and negative grid bias, employed with the modification of Fig. 6 for carrying out the method of this invention.

In Fig. la, at 4 is a thermionic pentode, having a cathode 4e, control-grid 4d, screen-grid 4c,

and plate 4a, the suppressor 4b being connected to the cathode. Plate 4a is connected to an output terminal 2, and is supplied with continuous voltage through a resistance 42, by the conductor Me, which leads to a source of continuous voltage such as 39, Fig. 1, and screen-grid 4c is connected to 41: through a resistance 4h. A condenser. serves to by-pass alternating current of the screen-grid to the output terminal 2a, and similarly the condenser 4p serves to by-pass alternating current of the high-potential side of the conductor Us to the terminal 2a. The terminal Zais groundedat 4y, At 4g is a heater element for the cathode 4e, one terminal being connected to 2a, and the other terminal 47' being connected to a source of power such as 44, Fig. l. The input terminal la is negatively biased with reference to the cathode 4e, by the resistance 4 the condenser 4g serving to by-pass alternating currents. It is understood thatan alternating voltage having amplitude-modulation is impressed across I-la, and similarly that the output terminals 4 2, 2a, supply an alternating voltage having amplitude-modulation, for the purpose of this invention.

In Fig. 3, an alternative form of output linkcircuit is shown. The inductance 45 either replaces or is connected in series with a resistance such as 41, by means of the terminals 2' and 2'0. Coupled with the inductance 45, by mutual inductance, is the inductance 46, tuned by condenser 48, and leading to the terminals 21) and 2e. Similarly coupled with 45 is the inductance 41, tuned by condenser 49, leading to the terminals 2c and 2d.

Referring to Fig, 1, at 5 is a thermionic tube including a triode with plate l0, grid l3, and cathode 9, and a pentode with plate I, suppressor 8, screen-grid l2, grid H, and cathode 16. In close proximity with cathode 15, is a diode-plate 6. The cathode I6 and cathode 9 are connected to .the terminals I 4, IS, the therminal [5 being connected through the switch 35 to "the power source 44, when 35 is closed, and through the resistance 31 and conductor 36 back to H. The

suppressor 8, is connected to l5. At 29 and 30, are high resistances, 29 being connected across the input terminals 2 and 2a by means of condenser 21 and condenser 28; resistance 30 being similarly connected by means of condensers 32 and 4|, it being understood that the condensers are of low impedance in relation to 29 and 30, the condensers 28 and 32 serving as stopping condensers for-continuous voltage upon terminal I, and thecondensers 21 and Ii serving as bypass condensers for carrying alternating currents of! and II respectively, directly to the terminal Is. One end of resistance 8| terminates at 2b.

the other end at 2e; similarly one endni 2'8 terminates at 2c, the other end at. For application oi amplitude-modulation to the circuit of Fig. 1, for the purpose of removing said modulation, the terminal 2 is connected to the terminal,

2 of Fig. 1a,- and likewise the terminal in is convoltage with amplitude-modulation, is thus pro vided between the terminals 2b and 2e, and between the terminals to and 2d.

The terminal 21) is connected to the grid it, of the triode l-ll-li. The plate ll is connected through the resistance 24 and conductor II to a source of continuous voltage 30, the negative terminal of I! being connected tothe cornmon conductor joining terminal Ia with to, The

resistance 14 is of the order oi magnitude of the I cathode-plate resistance, 9-", or larger, or the purpose of supplying an amplified amplitudemodulated alternating voltage in relation to the alternatingvoltage impressed upon it. At ll is a high resistance connected from it to the diode-plate l, the resistance I! being of such value in relation to the thermionic resistance l'll, that substantially direct proportionality exists between amplitudes of applied alternating voltage and rectified voltage across ll, in a manner well understood. At 240 isa variable connector for applying a selected amount of amplified amplitude-modulated alternating voltage to the diode-plate C, through the stopping condenser II. The condenser Ii has a low impedance relative to 24; the condenser 2| across I! is provided to cooperate with II to reduce the magnitude of fluctuations oi rectified-voltage, in relation to irequenciesoi the order oi magnitude of the carrier frequency. Unidirectional voltage is provided across H, which varies directly with the amplitudes of the amplitude-modulated alternating voltage across the terminals 2b-Ie, and which islarger than the respective amplitudes because of amplification of the triode l-lt-it.

Connecting the diode-plate I with the terminal Id oi the resistance II, is a choke coilil, one end of the choke coil being connected to the common connection-2H0 through the condenseril. The choke II and condenser It are pro-'- vided to substantially eliminate variations in the unidirectional voltage across ll of carrier frequencies, but to permit in the unidirectional voltage corresponding to frequencies of amplitude-modulation. This imidirectional voltage varying with frequency of amplitude-modu-- -lation is'impressed upon-the grid ll oi the pentode through theresistance II. and vthe potentionseter 38, which is supplied immithe source of continuous l voltage 38, when the switch 4. is closed. Both unidirectional voltage across i1 and potentiometer provide negative biasing oi the grid nection its-3a, which is grounded at 1/, The resistance 31 is proportioned to be very small in relation to the alternating current cathode-plate resistance, IB- I- of the pentode, so that sub-item nccted tothe terminal 2a oi. Fig, 1a. Alternating tially all of the alternating current resistance of the circuit l8'l.8lfl-IO is comprised within the thermionic pentode. Connecting II with the output terminal 3 is a condenser 41, and across the output terminals Ha is a high resistancejt. The resistance 26 is high in relation to 3|, and the, impedance of condenser 48 is negligible in relation to II, so that the alternatlng output voltage across 1-31: is substantially the same as the alternating voltageacross ii. The screen-grid l2 isconnected by way of the conductor 45 to the source of continuous voltage 89, and the condenser 20 serves to by-pass alternating currents of the screen-grid-dlrectly to Fig. 2. An important feature of this invention, i

is the utilization of a unique relationship iound to exist in numerous conventional pentodes, such for example as the pentodes at present manufactured and designated by numbers 78, .9003, 958, GSK'I, 6U'l-G, and others, wherein a fixed negative bias voltage -'E determines a reciprocalrelatlonship between grid transconductance and negative grid bias in excess of the fixed, negative bias, which exists'over a substantial and useful range of negative grid-voltages, many times the magnitude of the carrier voltage employed. Numerous tubes provide such a useful reciprocal relation over a range of 10 to 20 volts negative, in excess of a. predetermined negative bias, as 11- lustrated for example, between indices' ll .and BI. The solid line 54a indicatesa true rectangular-hyperbola with reference to the axis 54 defined by the specific negative grid bias E, and

relationship. Thus it will be noted that the, al-

ternating current flowing through the circuit l8-l3i-42I5i8 is determined by the; product of the grid transconductance' a and the varying amplitude oi the amplitude-modwlated alternating voltage impressed upon the The grid transconductance however, is made II. to vary in a reciprocal relationwith the excess negative bias, which in turn varies directly with the amplitudes of the modulated alternating voltage, so that the-product of any 01' the various alternating voltage amplitudes with their rupeetive resultant grid transeonductance is'constant. Thus the ratioof alternating current through 8! to the alternating current conductance of ii is constant, and there is a resultant alternating voltage of constant amplitude across ti. and 3 across the output terminals 3-30. In Hg. 4, a carrier voltage of amplitude c. with amplitudemodulation resulting in the various amplitudes e 2e 4e. to is shown. In Fig. 5, these voltages are shown in relation to the respective grid transconductance, determined by the negative unidirectional voltages Ke ,v 2Ke in and Ee resulting from the amplification of K times tion.' I

An alternative means of applying the amplitude-modulated alternating voltage to the terminals Ib-Ie, and to the terminals 20-, is illustrated in Fig. 3. In this instance, the resistances 2! and 30, condensers 32, II, 21, H, are not employed, the terminals Ib-Ie of Fig. 3 being 'the various amplitudes and subsequent rectificadirectly connected to 2b-2e oi Fig. 1, and the terminals 2c--2d of Fig. 3 being similarly connected to Zc-Id of Fig. 1.

Referring to Fig. 2, a modification of Fig. 1 is shown, employing a pentode a and a separate triode 5b. The input terminals for application of an amplitude-modulated alternating voltage are shown at 2b2'e, and at Z'c-I'd, it being understood that either a linkage-connection as shown in Fig. 1 at 2!, 30, 21, 28, 3!, I, using a resistance-condenser combination, or the arrangement of Fig. 3, can be employed. In using the linkage of Fig. 3, the terminal 2b is connected to 2'17, ie to 2'e, 20 to 2'0, and id to 2d.

Across the terminals 39a and 39b is a source of continuous voltage, and a resistance 39d, the continuous voltage being for the purpose of supplying the plate i lib, plate la, and screen-grid Ha, the negative terminal 39a being grounded. Across Sic-36b is a source or continuous voltage for supplying the heater elements 3 and ii, of triode 5b and pentode 5c respectively. Cine terminal of each heater element is connected to the common conductor Za-ia, the other is connected by 36 to the positive terminal 36!). Negative bias for the triode 5b is provided by means of the resistance 34b connected between the cathode 9a and common conductor 2a3a. The condenser 34a is provided to by-pass alternating currents of carrier frequencies. The triode 5b performs the same function of amplifying the impressed amplitude-modulated alternating voltage as in Fig. i, and likewise the pentode 5a performs in the same manner as the pentode of Fig. 1. A resistance 25a connects the diode-plate 6a with resistance H, the opposite end of resistance i! being connected to the cathode "a. Ampliiied modulated alternating voltage is impressed upon the diode-plate 6a through the stopping condenser 25. Provision is made at the terminals 33c and lid for the introduction of a fixed negative bias such as 33a, however the terminals 330 and 33d are connected together when a thermionic tube is employed at in capable of providing the requisite reciprocal relationship of grid transconductance with negative grid bias without fixed negative bias, and relative to zero grid bias.

Referring to Fig. 6, a further modification of apparatus is shown for carrying out the method of this invention, the larger conductance of the i circuit, corresponding to", Fig. 1 or Fig. 2, is in this instance the cathode-plate thermionic conductance of a triode, modified by rectified amplified amplitude-modulated alternating voltage to also maintain a constant ratio between alternating current and magnitude of alternating current conductance of the larger conductance. At lc is shown a pentode having cathode ilc, grid llc, suppressor 8c and plate 10. The suppressor In is connected to the cathode I80. At lib-39c is a source of continuous potential,

Y and cathode ll,

variable connector connected to resistance llc for applying a selected amount of amplified amplitude-modulated alternating voltage through the stopping condenser c to the diode-plate 6c, and to the resistance ilc which isconnected to the cathode "c. At 10 is a double-triode tube, one of the triodes having the plate 66, grid ll, the other triode having plate 61, grid l8, and cathode 12. The heater elements II and "of the cathodes l2 and II are connected together as shown, one terminal being connected to the output terminal 3a, which is grounded at ly, the other terminal being connected by the conductor 36 to a source of power such as indicated at 36b, Fig. 2. The cathode H is connected to 3a as shown. The plate I is supplied by a continuous voltage, for example by connection to resistance 39d through conductor 58a and resistance 58. Connected from the plate 65 to'the terminal 3a is a resistance 59. The triode 68-69--H' has a substantially straight-line relation between continuous current conductance of the cathode-plate i i-BB and negative grid bias voltage applied to grid 69, from across whichis the resistance 39d. The screengrid lIc is connected by conductor to the resistance 39d. The plate 1c is energized through a resistance sic connected by conductor 46 to the positive terminal ab. Input terminals for applying amplitude-modulated alternating voltage are shown at 2" and-2"a connected to the input resistance 200 through condensers 20c and lid, the grid llc being negatively biased by voltage drop, through the resistance Ill, caused by new of plate-cathode current Tit-"0, the com I denser II serving to by-pass alternating currents and having negligible impedance relative to any of the resistance elements. At 14 is a the rectified amplified modulated alternating voltage produced across lie. is negative relative to the grounded portion 41/ of the resistance 39d. One end of the bias resistance 60 is connected to adjustable connector 14 and to resistance 15, which resistance is shunted across the terminal 380 and ground connection 41 Thus, relative to the cathode II there are two opposite continuous voltages upon the grid 69, when there is no alternating carrier voltage across the terminals 2"2"a. The normal, and substantially constant operating bias of the pentode 50 which exists across 80, is opposed by a greater negative biasing voltage across". This is adjusted by means of H to give a resultant negative bias upon grid 89. When there is a rectified voltage across llc, this unidirectional voltage is superposed upon the resultant initial negative bias of grid 69, uniformly decreasing the continuous. current conductance Q of 66- in'relation to magnitude of rectified voltage across Ho. The conductances 58, and 59, are proportioned in relation to the thermionic conductance. "-1! so that as the thermionic conductance 88- uniformly decreases, the continuous potential difference across is uniformly increases, in a manner well understood. Connected from the plate 88 to the terminal la is a bypass condenser 59a for by-passing any variations in cathode-plate current through "-1! occasioned by variations in the unidirectional voltage across "c of carrier frequency. A condenser 15a across la and 14, of negligible impedance in relation to any of the circuit resistances, serves to by-pass alternating currents around I5. Adjustably connected to 59 by the connector 13 is the grid 68, the grid 68 being connected through the choke coil 85, and to the terminal 3a through the condenser 83. The choke O5 and condenser I operate to remove any residual fluctuations in the unidirectional voltage ofcarrier frequency. Connected from the positive conductor 48 to the terminal 3a, ishigh-resistance potentiometer "-62, the cathode 12 being connected by adjustable connector la to 82. The by-pass condenser 64 having negligible impedance in relation to cathode-plate impedance of 61|2, connects 12 with the terminal 3a. The plate 61 is supplied with continuous voltage from the conductor 48 through the high-impedance choke coils I4, a, the impedance of H and The terminal m 54a being very high in relation to the magnitudes of alternating current conductance employed between 61-12. The condenser 20, connected between screen-grid I20 and 3a serves to by-pass currents of carrier frequency. The condenser 42 connected between the terminal 3a and the conductor 46 serves to carry the alternating currents of the resistance 3lc directly to the terminal 3a. The plate 61 is connected through a resistance 51 stopping condenser 56 and variable connector 55 to the resistance 310. The resistance 51 is very high in relation to any magnitudes of alternating current resistance employed between 61-12, so that alternating currents traversing. the path l6c-1c-3lc-56-51- 61-12-64-15a-59-l6c are substantially unchanged by any modification employed in the thermionic conductance 61-12 when modulated carrier voltage is impressed upon 2"-2"a.

It will be noted that when there is no carrier voltage impressed across 2"-2"a, that relative to cathode 12 the grid 68 has a positive voltage across 59 opposed against a negative voltage across part of 62. The connector am is normally adjusted to provide negative bias upon grid 58 in excess of the positive bias across 59, sufficient to substantially cut off the cathode-plate current between 61-12 With carrier voltage present upon 2"-2"a, the grid 66 becomes correspondingly less negatively biased, and the alternating current conductance of E'l-12i correspondingly increased. The output terminal 3 is connected to the plate 6? through a stopping condenser 43, and connected to the terminal 3a through a resistance 26a. The resistance 26a is very high in relation to the resistance 61-12 employed between cathode-plate, and the impedance of condenser 33 is negligible in comparison with 26a; the alternating voltage across E-Ea is thus substantially the same as across iii-l2, and the resistance 26a also provides a conductive connection between 3 and 3a for use in a subsequent stage. The alternating voltage across 61-12 is thus determined by the ratio between the alterhating current flowing therethrough and the magnitude of the alternating current conductance of 61-12, each quantity or" which, through the structure described varies directly with the amplitudes of the amplitude-modulated voltage impressed upon 2-2a.

Referring to Fig. '2, a typical characteristic employed in this invention for the triode 61-68-12 is illustrated, relating cathode-plate alternating current conductance with negative grid bias. At i! is the position of zero grid bias, and at 6' is the position of cut on of cathodeplate current, occasioned by a negative bias of magnitude Ec. At 2g, fig, fig, are shown relative magnitudes of cathode-plate conductance for the alternating voltage amplitudes 6 26g, lc and fie of the modulated carrier voltage. The characteristic illustrated in Fig. '1, employed in this invention, is typical of numerous conventional triodes, such for example as those designated by numbers 6L5-G, 85, 605, and others. As employed in Fig. 8, the grid transconductance of the pentode is substantially con-- stant for the relatively small voltage amplitude of signal used, and with the substantially con stant negative bias used. The amplitudes of the alternating currents set up in the circuit iEc-lc-S 10-56-51-61- are thus proportional to the alternating voltages impressed upon 2"-2"a, and the conductance across 61-12 to these currents is also varied in direct proportion to said alternating voltages, providing a constant amplitude alternating voltage across 61-12, and across 3-3a.

In the circuits above described, the conditions providing a constant ratio between alternating current magnitude in the principal circuit noted in each instance, and the magnitude of the larger conductance, Whether the larger conductance is a thermionic conductor 61-12 as in Fig. 6, or a conductor 3| as in Fig. 1 or 2, are attainable with resultant gain of amplitude between the input terminals and the output terminals 3-3a. An important feature of the method is the smallness in variation of conductance over a cycle of impressed frequency. In Fig. 5, and Fig. 'l, the alternating voltages shown are large in relation to a range of negative bias voltages of ten to twenty volts, for example, in order to more clearly illustrate the principle; however in actual use the alternating voltages are relatively smaller than shown, and the conductance over a cycle is more nearly constant, thus providing alternating currents of carrier frequencies Without substantial alteration of wave-form, and without introducing extraneous frequencies. Another feature of importance, is the capability of removing amplitude-modulation when the impressed carrier voltage has a high percentage of amplitudemodulation. In terms of the ratio of highest amplitude to lowest amplitude, designated for example by p, the modulation index is v p 1 M P+ I the modulation index M being otherwise defined as the ratio of difference between extreme amplitude and the carrier amplitude, to the carrier amplitude, the latter being the arithmetic mean of the extreme amplitudes. Thus the larger the ratio p, the higher is the percentage modulation, M, e. g., if p:10, 1%:9/11, or about 82 percent amplitude modulation. The method of this invention permits of the employment of a large ratio between the highest and lowest amplitudes, as indicated in reference to the application of Fig. 5 and Fig. 7.

Having thus described several illustrative embodiments of my invention, it will be evident that changes can be made in the form and arrangement of parts without departing from the spirit of my invention, as set forth in the appended claims, and I do not therefore limit the scope of the invention to such particular embodiments, or otherwise than by the terms of the appended claims.

What is claimed is:

i. A modulation reducing device, for substantially removing amplitude modulation from an amplitude-modulated high-frequency voltage, said device having a thermionic pentode with cathode, plate, grid, screen-grid, and a suppressor connected to the cathode, capable of providing grid-transconductance variations in a reciprocal relationship with modifications of negative grid bias, power supply means for said cathode, continuous voltage means for said. plate and screengrid, an output conductance connected in series with the cathode-plate alternating current conductance, large in comparison therewith, amplifying means. independent of any alternating voltages across said output conductance, and across said cathode-plate, separately amplifying said modulated high-frequency voltage, diode means including said cathode, for rectifying said amplified modulated high-frequency voltage in direct proportion to the amplitudes of said highfrequency voltage, a nor -determining negative bias, for determining a range of negative grid voltages in excess of said bias which yields a substantially constant product of said excess bias voltage with respective magnitudes of grid-transconductance, said range being large in relation to the amplitudes of said modulated high -frequency voltage; said device having a series con- 4 nection between said grid and cathode, including said norm-determining bias, said modulated high-frequency voltage, and said rectified amplified high-frequency voltage in negative bias relationship, whereby voltage of said high-freuuency and substantially constant amplitude is provided across said output conductance.

2. The apparatus for removing amplitude modulation from an amplitude modulated altermating voltage, said apparatus having a thermiamplitudes of said alternating voltage, a primary negative bias for adjusting relationship between grid-transconductance and negative grid voltages in excess of said bias, and a secondary negative bias derived from said rectified alternating voltage, connected in series with said primary bias and said modulated alternating voltage, between the cathode and the grid, whereby alternating voltage of unmodulated amplitude is produced across said output conductance.

3. The combination-with a thermionic pentode having cathode, plate, grid, screen-grid, and a suppressor comiected to the cathode, capable of providing grid-transconductance variations in a reciprocal relationship with modifications of neg ative grid bias, of circuit means for removing am-- plitude modulation from an amplitude-modulatedalternating carrier voltage, said circuit means including input terminals for applying amplitude-modulated alternating voltage, output terminals for supplying constant-amplitude alternating voltage, continuous voltage means supplying said plate, and said screen-grid, amplifier means independently amplifying said amplitudemodulated alternating carrier voltage, independent of any alternating voltages across said cathode-plate, and across said output terminals, diode-means including said cathode, rectifying said amplified modulated carrier voltage in direct proportion to the amplitudes of said modulated carrier voltage, a norm-determining negative grid bias determining a range of negative grid voltages in excess of said bias which yields a substantially constant product of said excess bias with respective magnitudes of grid-transconductance, said range being large incomparison with amplitudes of the modulated carrier voltage,

an output conductance connected to said'output terminals, connected in series relation with the cathode-plate alternating current conductance,

5 and large in relation thereto, and a series connection between the cathode and grid, including said norm-determining bias, said rectified ampliiied modulated carrier voltage with negative gridbias polarity, and said modulated carrier voltage, whereby amplified carrier voltage with unmodulated amplitude is produced across said output conductance.

4. A modulation reducing device, for substantially removing amplitude-modulation from an amplitude-modulated alternating carrier voltage, said device having a thermionic pentode with cathode, plate, grid, screen-grid, and a suppressor connected to said cathode, an alternating current output conductance connected in series relation with the cathode-plate conductance of said pentode, and large in relation thereto, said alternating current output conductance comprising the cathode-plate alternating current conductance of a thermionic triode, having cathode, grid and plate, capable of uniformly decreasing in magnitude with increase of negative bias upon said triode-grid, continuous voltage supply for the plate and screen-grid of said pentode, and the plate of said triode, amplifying means including.

said pentode, independently amplifying said amplitude-modulated carrier voltage, independent of any alternating voltages across said output conductance, diode rectifying means including the cathode of said pentode, rectifying said independently amplified modulated carrier voltage to a unidirectional voltage directly proportional to the amplitudes of said amplitude-modulated carrier voltage, negative bias means for biasing the grid of said triode to substantially cut oil! the cathode-plate current of said triode, and a polarity-conversion means impressing said unidirectional voltage upon the grid of said triode in series relation with said negative bias means,

to differentially positively bias said triode-grid in direct proportion to the amplitudes of said amplitude modulated carrier voltage, whereby a substantially constant amplitude alternating carrier voltage is established across said output conductance.

5. A modulation removing circuit for removing amplitude modulation from an amplitude-modulated alternating voltage, said circuit having an alternating current input conductance connected in aseries relation with an alternating current output conductance, said input conductance including thermionic amplifying means independently amplifying said amplitude-modulated alternating voltage, independent of any alternating voltages across said output conductance, said output conductance being large in relation to said input conductance, and consisting of the cathode-plate alternating current conductance of a thermionic triode, having cathode, grid, and

plate, capable of uniformly decreasing with increase of negative bias of said triode-grid, a continuous voltage supply for the plate of said triode, diode rectifying means connected with said amplifying means, providing a unidirectional voltage in direct proportion to the amplitudes of said modulated alternating voltage, from said amplified modulated alternating voltage, negative bias means for setting said triode grid to substantially cut oflf plate current of said triode, and bias opposing means for opposing said unidirectionalvoltage against said negative bias, whereby said output conductance is varied in direct proportion to the amplitudes of the amplitude-modulated alternating current through said conductance, and alternating, voltage of constant amplitude is provided across said-output conductance.

6. The apparatus for removing amplitude modulation from an amplitude-modulated alternating input voltage, said apparatus having a first thermionic tube with cathode, plate and grid,

, and an auxiliary plate associated with said cathode, an alternating current output conductance connected in a series relation with the cathodeplate alternating current conductance of said first thermionic tube, and large in comparison therewith, consisting of the cathode-plate alternating current conductance of a second thermionic tube, having cathode, plate and grid, continuous voltage means for supplying the plates of said first and second tubes, amplifying means including said first thermionic tube, independently amplifying said modulated alternating input voltage, independent of any alternating voltages across said output conductance, rectifying means including said auxiliary plate and associated cathode converting said independently amplified modulated alternating input voltage to a unidirectional voltage, in direct proportion to the amplitudes of said modulated alternating voltage, a primary negative bias for adjusting the grid of said second tube to substantially cut off the plate current of said tube, and a positive secondary bias connected in a series relation with said primary bias, said secondary bias being derived from said unidirectional voltage, and directly proportional thereto, whereby a constant ratio is maintained between the magnitude of the alternating current through the output conductance and the magnitude of said altematins current output conductance, independent of variations in amplitude of said amplitude-modulated alternating voltage.

7. A circuit for removing amplitude modulation from an amplitude-modulated alternating voltage, said circuit having two alternating current conductancesin a series, relation, one of said ,conductances being large in comparison with the other, one of said conductances consisting of the cathode-plate alternating current conductance of a thermionic tube having cathode, plate and grid, continuous voltage means energizing said plate, input-terminal means impressing an amplitude-modulated alternating voltage upon said conductances, rectifying means energized independent of any alternating voltages across said cathode-plate, converting a large multiple of said modulated alternating voltage into a unidirectional voltage directly proportional to the amplitudes of said modulated alternating voltage, and bias means connected to said grid, having a primary negative bias in series relation with a secondary bias, said primary bias determining the magnitude of the cathode-plate conductance when no alternating current flows through said conductance, and said secondary bias being derived from said unidirectional voltage, determining conductance of said thermionic tube in relatlon to the amplitudes of said modulated alternating voltage, whereby a constant ratio is maintained between the magnitude of said larger conductance and the magnitude of the alternating current through said larger conductance, to produce therefrom an alternating output voltage of constant amplitude, across said larger conduct once.

8. The method of annulling amplitude modulation of an amplitude-modulated alternating voltage, said method. comprising introducing a. proportion of said voltage in scrim relation with a first electric-conduction path serially connected with a second electric-conduction path, proportioning said second conduction path to have large conductance in relation to said first conduction path, independently amplifying .said modulated alternating voltage, independent of any alternating voltages across said second con-.

duction path, rectifying said amplified modulated alternating voltage in direct proportion .to the 10 amplitudes of said modulated alternating voltage, combining said rectified voltage with a fixed unidirectional voltage, and electrostatically introducing said combined unidirectional voltages across a part of one of said electric-conduction paths, to modify the electric conductance of said path in a predetermined relation with the amplitudes of said modulated alternating voltage, whereby a constant ratio is established, between the amplitude of alternating current and the magnitude of conductance of said second path,

and alternating output voltage of constant omplitude is established across said second conduction' path.

9. The method of annulling amplitude modulation from an amplitude-modulated alternating voltage, which consists of amplifying said modulated alternating voltage, rectifying said amplifled modulated alternating voltage in direct proportion to the amplitudes of said modulated alternating voltage. serially combining said rectified voltage with a fixed unidirectional voltage, and electrostatically introducing said combined voltage across part of a thermionic conductance, modifying said conductance in a. prede relation to the amplitudes of said modulated alternating voltage, introducing a proportion of said amplitude-modulated alternating voltage in a series relation with a, second conductance serinlly connected with said thermionic conductance. to andproportionlng the ratio of said thermionic conductance to the second conductance to have a large disparity with unity, whereby a constant ratio is established'between the amplitude of alternating current and the magnitude of the larger conductance, and alternating output volt-" age of constant amplitude established across said larger conductance.

10. In a device for transforming variable-amplitude alternating input voltages to constant- 0 amplitude alternating output voltages, alternatlug-voltage input terminals, alternating-voltage output terminals, thermionic-tube conductance .means including cathode, anode and controlgrid; independent amplifier means providing omplification of alternating input voltages, independent of the magnitude of any alternating voltages across said output terminals, and scroll cathode-anode of said conductance, converter means including a rectifier-anode and connections to said amplifier means, converting said amplified voltages to unidirectional voltages in direct proportion thereto, connections between said output terminals and the cathode and mode of said conductance, and biasing means acre. said cathode and control-grid having a primary voltage-bias means serially connected with a secondary voltage-bias means, said primary bias dotermining amount of conductance of said cathode-anode when said input terminals are not 70 energized, and said secondary bins being responsive to the unidirectional voltage of said converter means, said bias means being polarized in relstion to each other and to the chenct'uilflc conductance of said cathode-anode to provide 15 constant amplitude alternating output volts...

2,824,090 independent of amplitude variations of the alternating input voltage.

11. A circuit for removing amplitude modulation from an amplitude-modulated alternating input voltage-said circuit having two alternating current conductances in a series relation, one of said conductances being. proportioned to be large in relation to the other, one or said conductances including the cathode-plate conductance 01' a thermionic ,tube having cathode, plate and grid; continuous voltage means energizing said plate, input-terminal. means impressing amplitudemodulated high-frequency voltage upon said conductances, output-terminal means supplying constant amplitude voltages of said high-frequency,

rectifying means responsive to amplitude-modulated voltage or said input terminals, independout or any alternating voltages across said outrent flows therethrough, and said secondary bias being responsive to said unidirectional voltage, said bias means being polarized in relation to each other and-to said cathode-plate conductance, whereby a constant ratio is maintained between the magnitude of said larger conductance and the alternating current therethrough.

EDWARD H. LARGE. 

